From The Limits of Safety (p. 1):

On the night of October 25, 1962, an air force sentry was patrolling the perimeter of a military base near Duluth, Minnesota. It was the height of the Cuban missile crisis, and nuclear-armed bombers and interceptor aircraft, parked on air base runways and at commercial airports throughout the United States, were alert and ready for war. The sentry spotted someone climbing the fence, shot at the figure, and sounded the sabotage alarm. At airfields throughout the region, alarms went off, and armed guards rushed into the cold night to prevent Soviet agents from sabotaging U.S. nuclear forces.

At Volk Field in Wisconsin, however, the wrong alarm bell rang: the Klaxon signalling that nuclear had begun went off. Pilots ran to their nuclear-armed interceptors and started the engines. These men had been told that there would be no practice alert drills during the tense crisis, and they fully believed that a nuclear war was starting as they headed down the runway. Fortunately, the base commander contacted Duluth before the planes took off and discovered what had happened. An officer in the command post immediately drove his car onto the runway, flashing his lights and signaling the interceptors. The pilots saw him and stopped their aircraft. The suspected Soviet saboteur that caused the whole incident was, ironically, a bear.